Among biodegradable resins, polylactic acid resins are now expected to expand applications, because a production cost of L-lactic acid has been reduced by a high-volume production thereof by fermentation of sugars derived from corn and potato, these raw materials are natural agricultural crops and emit a very small amount of carbon dioxide, and the produced resins have high rigidity and good transparency. Polylactic acid resins are, however, brittle and rigid materials lacking flexibility. Due to such characteristics, there are a wide variety of applications of these resins in consumption commodities such as packaging materials and daily necessaries but scarcely any in the field of durable items that are required to satisfy high demand characteristics such as injection-molded parts for home electronics and vehicles. Injection-molded parts of polylactic acid resins have problems of insufficient flexibility, impact resistance, and mechanical strength, whitening in bending, and low hinge characteristics, and are useless under present circumstances.
Polylactic acid resins also have a disadvantage of low crystallization speed, and are in an amorphous state after molded unless subjected to mechanical treatments such as stretching. However, polylactic acid resins have low glass transition temperature (Tg) as around 60° C. and low heat resistance, and have a problem of being unusable under conditions of 55° C. or higher.
Further, if a polylactic acid resin is used in durable items such as apart for home electronics and vehicles, the resin must have flexibility at a certain level as well as heat resistance and impact resistance.
JP-A-2005-120119 discloses a flame retardant injection-molded product having both flame retardancy and durability, containing a lactic acid resin, a metal hydroxide surface-treated with a silane coupling agent, and an aromatic carbodiimide. This patent describes that a preferred silane coupling agent is epoxysilane and there are only examples using an epoxysilane coupling agent. Use of a metal hydroxide surface-treated with an epoxysilane coupling agent, however, achieves insufficient effects. Therefore, there is still a demand for developing a polylactic acid resin composition having high impact resistance and flexibility.